I. FIELD OF THE INVENTION
This invention relates to a method of manufacturing carbon substrates suitable for substrates of magnetic disks, magnetic heads, optical lens molds, optical reflectors, LSI package and photosensor drums and the like.
II. DESCRIPTION OF THE PRIOR ART
A method of manufacturing carbon substrates as is used in this field is to hot mold and burn resin powders, then with or without hot isostatical pressure treatment to polish substrate surfaces and to produce mirror-finishing surfaces. The well known method for such hot molding process, specifically, proceeds to fill powders of phenol-formaldehyde resin into a die, to apply a pressure 100 to 200 kgf/cm.sup.2 (for example, 150 kgf/cm.sup.2), and to mold during 30 to 60 minutes (for example, 45 minutes) at a temperature of 150.degree. to 170.degree. C. (for example, 160.degree. C.) (see Japanese Patent Application Laid open No. Sho-62-234232 in 1987).
However in the conventional hot molding method for the carbon substrates of these kinds, void or pore generated on molding comes to substrate defect after completion of the carbon burning or the surface polishing. Further in those conventional methods, a good amount of gas generates to require a long time for molding, and thereby disadvantageously results in a higher production cost. On filling the powders into the die, hardening begins from a portion in contact with the mold surface due to a high temperature of the mold; this always starts to harden the initially inserted powders, and degrades uniformity of the molded body. Such ununiform molding causes defects of the substrates, requires much time for cleaning the die with a trouble some of complicated handing, and provides difficulty in the continuous molding process.
In the substrates for the magnetic disks which are essentially frequently used in the carbon substrates, strong demands are directed to improvement of surface precision and reduction of surface defects depending on rapid development of recent magnetic disk devices and realization of a high dense recording of the magnetic disks. Amorphous carbon material exhibits property of graphite dispersed into matrix of amorphous carbon. To polish the carbon substrate for mirror finishing, a partially concentrated removal by polishing is performed, such removed portion being of graphite as a core, a largeness of the cavity portion sometimes is larger than a diameter of graphite portion. As described in problem, the conventional method has a possibility to enlarge the defect and to increase the number of defects per unit area as a magnetic disk substrate.
The conventional method of polishing and mirror finishing the carbon substrate, which is capable of eliminating such increase of the number of defects, is to use a tin wheel, and to one-side lap the carbon substrate by aqueous solution in which diamond abrasive grain is dispersed. With this method employed, it is possible that a largeness of void generated on the substrate surface is made substantially equal to a diameter of graphite.
However, the conventional method which uses the tin wheel and one-side laps the carbon substrate by aqueous solution in which the diamond abrasive grain is dispersed, may not be preferable because it requires an extremely high cost for the abrasive grain and therefore requires further a higher operation cost when employing such mirror finishing method for a mass production system. In problem, the method, which is difficult to produce a larger capacity and scale, hardly realizes a larger amount of treatment at the limited time.
Polishing by etching-liquid is widely utilized because the amount of treated alternated layers is reduced by etching-effect, and improves polishing efficiency. For example, alkaline solution such as NaOH and KOH, which is used in polishing the silicon wafer, obtains an efficiency by several times the efficiency of the mechanical polishing (see KIKAI SINKOU KYOUKAI GIJUTU KENKYUUSHO, 0.5 Lapping and Polishing 55/75" in "Machining Technique Data Book").
Since carbon is an extremely stable substance with a high chemical-resistance, the amorphous carbon substrate does not exhibit any etching effect even in using the conventional mechanochemical polishing solution.
In the conventional method of burn-carbonizing the carbon substrate, thermosetting resin molded in a doughnut shape is burned in an inert gas atmosphere at a temperature of about 1000.degree. C. to be carbonized (see the Japanese Patent Application Laid open Number Sho-60-35333 in 1985 and Number Sho-62-23423 in 1987).
However, the thermosetting resin is contracted by about 20% by burn-carbonizing. This provides deformation of the molded body on such burning process, and generates a larger warping. The warping is normally removed at a certain extent by polishing. However, the recent development of the high-dense recording magnetic disk requires an extremely high-polished surface and a graded flatness degree, therefore, for a high dense recording carbon substrate, it is difficult to practically use such carbon substrate as manufactured by the conventional method. In burning process, if a jig to be used contains metallic impurities, these impurity elements are dispersed into materials during burn-carbonizing. In this manner, contaminated material is surface polished so that the contaminated portion comes to substrate defects such as pitting holes etc.
On the other hand, resin molded bodies after burn-carbonizing, which are in lamination each other inserted spacers between each resin molded body, are subjected to a hot isostatical pressure treatment (HIP treatment) to produce a high density thereof. The HIP treatment contracts the resin molded body by about 5%, and raises a material density up to about 1.55 to 1.8.
However, the contraction during the HIP treatment deforms the material and a large warping tends to arise in the obtained carbon substrate. The warping can be removed up to a certain extent by polishing, however, since the recent high-dense recording magnetic disk requires an extremely flat surface, therefore, it is difficult to practically use such carbon substrate.
If the jig used on HIP treatment contains the metallic impurities, these impurity elements are dispersed into the resin molded bodies during the burn-carbonizing. This results in a cause of the substrate defect.